The microRNA molecule miR-34a is a key suppressor of osteoclast development, bone resorption, and the bone metastatic niche, and it could prove to be an important target for new therapies in osteoporosis and cancers that metastasize to the bone, researchers reported.

When Yihong Wan, PhD, of the University of Texas Southwestern Medical Center in Dallas, and colleagues examined several microRNAs that have been found to be related to cancers during a time course of bone marrow osteoclastogenesis assay, they determined that miR-34a blocked the development of osteoclasts, while the related microRNAs miR-34b and miR-34c did not.

MiR-34a was rapidly downregulated by receptor activator of nuclear factor kappa-B ligand (RANKL) and further diminished by rosiglitazone, while levels of miR-34b/c were unaffected and expressed at much lower levels than miR-34a, they wrote in a research letter in Nature.

MicroRNAs are small, noncoding molecules that have been shown to play a critical role in cellular function. Studies are underway to determine if several microRNAs, including miR-34a, suppress primary tumors in humans.

These findings are among the first to suggest a role for a microRNA in the suppression of bone metastasis and osteoporosis, Wan told MedPage Today.

"This tiny microRNA could potentially lead to new types of medicines for not only treating primary and metastatic cancers, but for protecting bone and treating osteoporosis and other bone degenerative diseases," she said.

"Consistent with prior reports, miR-34a deletion had no overt effect on mouse development," the researchers wrote. "Osteoclast differentiation was augmented in 34a-Het and 34a-KO cultures, whereas precursor proliferation or survival was unaffected. As a result, serum carboxy-terminal telopeptides (CTX)-1 and osteoclast number were elevated."

The authors reported that global miR-34a deletion also decreased bone formation as the serum marker amino-terminal propeptide of type I procollagen (P1NP) was reduced, along with a reduction in osteoblast number, bone formation rate, and mineral apposition rate. The increased resorption in 34a-Het indicated that miR-34a function was haploinsufficient and sensitive to dosage reduction, they explained.

Using an ovariectomy (OVX) mouse model and a chitosan (CH) nanoparticle vehicle, the researchers were able to show that compared with sham controls, OVX mice treated with miR-Ctl-CH had increased CTX-1 and decreased P1NP, whereas both effects were largely prevented in OVX mice treated with miR-34a-CH.

"Consequently, OVX-induced bone loss was attenuated by miR-34a-CH," the group wrote. "miR-34a also decreased bone resorption and increased bone formation in sham controls, leading to a higher bone mass. Biodistribution analysis showed that miR-34a level in the bone marrow was highest and further increased fivefold by miR-34a-CH, indicating an efficient miR-34a delivery."

These and other studies in mice led the researchers to conclude that osteoclastic miR-34a overexpression is sufficient to impede osteoporosis, and the osteoclast is the key site for miR-34a therapeutic benefit.

In other studies, the researchers also showed bone metastasis in breast cancer and melanoma to be diminished in osteoclastic miR-34a transgenic mice. In both cancer models, bone metastases were attenuated in by miR-34 nanoparticle treatment.

Tgif2: Direct miR-34 Target

The researchers identified transforming growth factor -beta-induced factor 2 (Tgif2) as an essential direct miR-34a target. Tgif2 expression was found to be suppressed by miR-34a gain-of-function, but increased by miR-34a loss-of-function, both in mouse and human osteoclast cultures.

Wan said the roles of microRNAs in bone physiology are just beginning to be explored and understood.

"There haven't been many studies looking at microRNA in osteoporosis," she said. "Ours is among the first showing both a genetic and pharmacological approach."

She noted that several studies have demonstrated osteoblast-specific gain-of-function of miR-34b/c decreases bone mass by suppressing osteoblastogenesis and bone formation. The studies by Wan and colleagues show that osteoclast-specific miR-34a gain-of-function protects bone by suppressing osteoclastogenesis and bone resorption.

"Our study paves the road for future discovery of other miRNAs that may be regulated by RANKL and/or control Tgif2 expression, as well as future epidemiological and clinical studies to explore the pathological and therapeutic roles of this miR-34a-Tgif2 pathway in humans," the researchers wrote.

miR-34a Exerts Anabolic Effects

Most systemically delivered drugs can target multiple tissues and cell types. The work by Wan and colleagues showed that miR-34a also enhances bone formation; although miR-34a overexpression in osteoblast is not essential for the therapeutic benefits of miR-34a in osteoporosis or bone metastasis.

The researchers concluded that miR-34a may prove to be a novel therapy that exerts both anti-catabolic and anabolic effects compared with the current drugs that are solely anti-catabolic. They concluded that the identification of miR-34a, as well as a recent report of miR-141 and miR-219, opens a new avenue for the development of a new generation of RNA-based osteo-protective medicine.

Other miR-34a target genes have been reported in different biological context, such as SIRT1, SIRT6, and PNUTS.

"Although miR-34a may also target genes other than Tgif2 in osteoclasts, our genetic rescue ex vivo and in vivo shows that Tgif2 is the key miR-34a target, suggesting that other genes are probably secondary or functionally irrelevant to osteoclastogenesis," Wan and colleagues wrote.

miR-34a, miR-34b, and miR-34c are commonly deleted in human cancers. In vitro studies suggest that they may be critical mediators of p53 function and potential tumor suppressors. In vivo studies have shown that miR-34abc triple knockout mice exhibit intact p53 function without increased tumorigenesis.

The researchers noted that the finding that systemic miR-34a administration can attenuate cancer malignancy raises the "intriguing possibility" that its anticancer effects may reside in other cells that constitute the tumor microenvironment such as the osteoclasts in the bone metastatic niche.

"Indeed, our findings illustrate that bone metastasis are effectively blocked by miR-34a in osteoclasts, thus providing the first in vivo genetic evidence that miR-34a opposes malignant progression of cancer by disarming the metastatic niche," they wrote.

This research was funded by the Cancer Prevention Research Institute of Texas, the NIH, the Welch Foundation and the University of Texas Southwestern.

The researchers disclosed no relevant relationships with industry.

Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania

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